1. Field of the Invention
The present invention relates to an apparatus for performing radiation-assisted processing; more particularly, an apparatus for performing ultra-violet (UV) assisted cleaning of surfaces.
2. Description of the Related Art
Several types of radiation-assisted processing methods are currently in use, including rapid thermal processing (RTP), chemical vapor deposition (CVD), and UV/ozone cleaning. Rapid thermal processing is often performed using a radiation source which produces infrared (IR) heating, for example tungsten-halogen lamps which emit radiation having 70-80% of its wavelengths in the infrared range. Chemical vapor deposition can be accomplished using either mainly IR heating or by utilizing UV radiation in addition to IR radiation in order to speed up thermal reactions.
UV/ozone cleaning is performed using light sources which produce ultraviolet light, for example a lowpressure mercury lamp. Other uses for a processing apparatus producing ultraviolet radiation include photoresist removal and EPROM erasing. As used herein, radiation refers to electromagnetic radiation (or light) having wavelengths in the visible and near-visible range, including, for example, ultraviolet and infrared radiation.
The apparatus used for these various types of radiation-assisted processing are similar, and an apparatus designed for one type of radiation-assisted processing can easily be modified for use in another type of radiation-assisted processing by, for example, changing the radiation source. The UV/ozone cleaning process and an apparatus for performing same will be used, by way of example, to describe a conventional radiation-assisted processing apparatus.
In UV/ozone cleaning, a process which has been shown to be useful in the cleaning and preparation of, for example, silicon substrates, a substrate is placed in a reaction chamber having an oxygen containing ambient (for example, filtered air) and exposed to ultraviolet radiation. The ultraviolet radiation source is selected so that it produces radiation having wavelengths of 184.9 nm and 253.7 nm. The 184.9 nm wavelength is important because it is absorbed by oxygen, and it thus leads to the generation of ozone. The 253.7 nm radiation is not absorbed by oxygen; it therefore does not contribute to ozone (O.sub.3) generation. However, 253.7 nm radiation is absorbed by many organic materials and also by ozone, as well as many products of the reaction of ozone with organic materials. The absorption of 253.7 nm radiation by ozone is principally responsible for the destruction of ozone in the reaction chamber. Therefore, when both wavelengths are present, ozone is continually being formed and destroyed. An intermediate product of both the formation and destruction of ozone is atomic oxygen, which is a very strong oxidizing agent. Radiation of 253.7 nm is also absorbed by organic materials and their ozonolysis products and causes a decomposition of the organics.
The UV/ozone cleaning process can be varied by providing ozone to the reaction chamber from an external source rather than generating the ozone with ultraviolet radiation.
The efficiency, and thus the time necessary for UV/ozone cleaning depends on the ambient atmosphere, the intensity of the ultraviolet radiation, the temperature of the material being cleaned, and the chemical composition and structure of the material being cleaned, the latter either providing a catalytic effect or extra stability. The intensity of ultraviolet radiation emitted from a discrete source, or a plurality of discrete sources, at the surface of a substrate to be cleaned can be increased by decreasing the distance between the source Of the radiation and the substrate. However, it is beneficial to maintain a uniform flow of the ambient gas over the surface of the substrate to be cleaned, requiring that a gap remain between the substrate and the radiation source.
UV/ozone cleaning is discussed in the following papers: Uv/ozone Cleaning of Surfaces, John R. Vig, J. Vac. Sci. Technol. A, Vol 3, No. 3, May/June 1985; UV/ozone Cleaning of Silicon Substrates in Silicon Molecular Beam Epitaxy, Michiharu Tabe, Appl. Phys. Lett., Vol 45, No. 10, Nov. 15, 1984; UV/ozone Processing: Its Applications in the Hybrid Circuit Industry, F. K. Clarke, Hybrid Circuit Technology, Dec. 1985, page 42; and Dry Cleaning of Contact Holes Using Ultraviolet (UV) Generated Ozone, Norstrom, et al., J. Electrochem. Soc.: Solid-State Science and Technology, Vol. 132, No. 9, Sept. 1985, page 2285.
All radiation-assisted processing apparatus known to the inventor of the present invention which are capable of processing large substrates utilize a reflector to increase the intensity of the radiation used for processing. In particular, a reflector is used to redirect toward the substrate radiation which is otherwise directed away from the substrate by the radiation source. For example, in U.S. Pat. No. 4,558,660, reflectors are utilized for both ultraviolet and infrared producing radiation sources. U.S. Pat. No. 3,801,773 and the above-mentioned article entitled UV/ozone Cleaning of Surfaces illustrate some of the complexities and difficulties associated with the use of reflectors to enhance the radiation intensity at the surface of the substrate to be cleaned in order to reduce processing time.
Various materials have been utilized as reflectors for ultraviolet and infrared radiation and a great deal of time and effort has been spent in developing appropriate reflectors for radiation-assisted processing. Several problems are associated with reflectors, including the poor reflectivity of most metals, including gold and silver, in the ultraviolet range and the difficulty in preventing corrosion of a reflector subjected to UV and/or IR radiation. Aluminum is the preferred reflector; however, great efforts must be expended to prevent corrosion of the aluminum surface.
Furthermore, there are no 100% reflectors, and thus radiation (light) having an intensity I.sub.i when incident upon a reflector is reflected with an intensity I.sub.r, where I.sub.r is always less than I.sub.i. In addition, since the radiation source is situated between the substrate to be processed and the reflective surface a certain amount of the reflected radiation having intensity I.sub.r must pass through the radiation source before being incident on the substrate, causing a further decrease in the intensity of the reflected radiation. The reflected radiation which passes back through the radiation source has an intensity I'.sub.r, where I'.sub.r &gt;I.sub.r. The problem is illustrated in FIG. 1 in which radiation source 10 emits radiation in all directions. Some of the radiation is directly incident upon a substrate 14 supported on surface 12 and some of the radiation is directed toward reflector 16. Radiation emitted by radiation source 10 removed from the substrate 14 by a given distance has intensity I.sub.0 at the surface of substrate 14. Three paths for radiation emitted by radiation source 10 are illustrated by I.sub.i1, I.sub.i2, and I.sub.i3 in FIG. 1. I.sub.i1 is radiation which passes directly from radiation source 10 to substrate 14 and strikes substrate 14 with intensity I.sub.i. Radiation I.sub.i2 is reflected by reflector 16, has a reflected intensity I.sub.r2 (I.sub.r2 &lt;I.sub.i2), and then suffers a further reduction in intensity by passing back through radiation source 10 before striking substrate 14 with an intensity I'.sub.r2, where I'.sub.r2 &lt;&lt;I.sub.i2. Radiation I.sub.i3 is reflected by reflector 16 but does not pass back through radiation source 10, and thus strikes substrate 14 with intensity I.sub.r3, again I.sub.r3 &lt;I.sub.i3. The intensity of radiation reflected by reflector 16 which is incident on substrate 14 varies between approximately 50% and approximately 75% of the intensity of radiation I.sub.0 which is directly incident on substrate 14.
Moreover, as discussed above, ultraviolet radiation is absorbed by ozone. Therefore, as the ultraviolet radiation travels a greater distance before striking substrate 14, it suffers a reduction in intensity. The intensity decreases exponentially in accordance with Beer's absorption law: I.sub.x =I.sub.o e.sup.-ax, where x is the distance traveled by the radiation through an absorbing medium having an absorption coefficient a. Accordingly, the intensity of reflected radiation is always more greatly attenuated than the intensity of radiation traveling directly from a radiation source 10 to the substrate 14 because the reflected radiation travels an added distance equal to two times the distance between the radiation source 10 and reflector 16.